This lock was at the junction of the Somersetshire Coal
Canal with the Kennet and Avon Canal. Both canal companies had gone
to considerable expense to aquire water for their canals and they
were not willing to let this valuable commodity flow freely from one
canal to the other. Although the level of the S.C.C. was only a few
inches higher than the level of the K&A, this lock was installed
to separate the canals.

The lock was originally built 14 feet wide, which allowed
full-sized barges from the K&A Canal to have access to the lower
part of the S.C. C. as far as the flight of narrow locks above
Midford. Later it was rebuilt to a width of 7 feet, so as to only
allow narrow boats to enter the S.C.C. The plan below shows the shape
of the current lock and how it may have been originally built.

Construction

There is only one gate at each end of this lock - which is unusual
for the S.C.C. All the other locks on the canal have a single top
gate and double bottom gates; this is because a single bottom gate,
which is much deeper than the top gate, would be heavy to operate.
Furthermore, it would need more space in which to swing open, making
the lock chamber longer and wasting more water.

It is thought that the main reason for this lock having single
gates is that, when it was rebuilt from a double lock with double
gates, only one gate of each pair was retained. Because the
difference in water level between the two canals is so slight, only a
trivial amount of water is lost despite the extra length taken up by
having a single bottom gate.

Unusuallly, this lock has a third gate, which is in the middle of
the chamber and swings the opposite way from the other two. This was
a 'stop' gate' which would be swung shut by rapid movement of the
water from the K&A if the S.C.C. became breached and its level
suddenly dropped.